There have been many objections to the possibility of time travel1. But all the truly interesting ones concern the possibility of reverse causation2. What is objectionable about reverse causation3?

I diagnose that the truly interesting objections are to a further possibility: that of causal loops. I raise doubts about whether there must be causal loops if reverse causation4 obtains; but devote the majority of the paper to describing, and dispelling concerns about, various kinds of causal loop.

In short, I argue that they are neither logically nor physically impossible. The only possibly objectionable feature that all causal loops share is that coincidence is required to explain them. Just how coincidental a loop will be varies: some are really quite ordinary, and some are incredibly unlikely.

I end by speculating that the tendency amongst physicists to avoid discussion of causal loops involving intentional action may have been unfortunate, since intentional action is an excellent way to non-mysteriously bring about what otherwise would have been an unlikely coincidence. Hence causal loops may be more likely in a world with beings like us, than in one without.